GNU GRUB (Legacy) is a multiboot bootloader maintained by the GNU Project. It was derived from GRUB, the GRand Unified Bootloader, which was originally designed and implemented by Erich Stefan Boleyn.

Briefly, the bootloader is the first software program that runs when a computer starts. It is responsible for loading and transferring control to the Linux kernel. The kernel, in turn, initializes the rest of the operating system.

Finding GRUB's root

GRUB must be told where its files reside on the system, since multiple instances may exist (i.e., in multi-boot environments). GRUB files always reside under /boot, which may be on a dedicated partition.

Hard disks are defined as (hdX); this also refers to any USB storage devices.

Device and partitioning numbering begin at zero. For example, the first hard disk recognized in the BIOS will be defined as (hd0). The second device will be called (hd1). This also applies to partitions. So, the second partition on the first hard disk will be defined as (hd0,1).

If you are unaware of the the location of /boot, use the GRUB shell find command to locate the GRUB files. Enter the GRUB shell as root by:

# grub

The following example is for systems without a separate /boot partition, wherein /boot is merely a directory under /:

grub> find /boot/grub/stage1

The following example is for systems with a separate /boot partition:

grub> find /grub/stage1

GRUB will find the file, and output the location of the stage1 file. For example:

grub> find /grub/stage1

(hd0,0)

This value should be entered on the root line in your configuration file. Type quit to exit the shell.

Dual booting with Windows

Add the following to the end of your /boot/grub/menu.lst (assuming that your Windows partition is on the first partition of the first drive):

/boot/grub/menu.lst

title Windows
rootnoverify (hd0,0)
makeactive
chainloader +1

Note: If you are attempting to dual-boot with Windows 7, you should comment out the line makeactive.

Note: Windows 2000 and later versions do NOT need to be on the first partition to boot (contrary to popular belief). If the Windows partition changes (i.e. if you add a partition before the Windows partition), you will need to edit the Windows boot.ini file to reflect the change (see this article for details on how to do that).

If Windows is located on another hard disk, the map command must be used. This will make your Windows install think it is actually on the first drive. Assuming that your Windows partition is on the first partition of the second drive:

Dual booting with GNU/Linux

Note: There may be other options that are required, and an initial RAM disk may not be used. Examine the other distribution's /boot/grub/menu.lst to match boot options, or see #chainloader and configfile (recommended).

chainloader and configfile

To facilitate system maintenance, the chainloader or configfile command should be used to boot another Linux distribution that provides an "automagic" GRUB configuration mechanism (e.g. Debian, Ubuntu, openSUSE). This allows the distribution to manage its own menu.lst and boot options.

The chainloader command will load another bootloader (rather than a kernel image); useful if another bootloader is installed in a partition's boot sector (GRUB, for example). This allows one to install a "main" instance of GRUB to the MBR and distribution-specific instances of GRUB to each partition boot record (PBR).

The configfile command will instruct the currently running GRUB instance to load the specified configuration file. This can be used to load another distribution's menu.lst without a separate GRUB installation. The caveat of this approach is that other menu.lst may not be compatible with the installed version of GRUB; some distributions heavily patch their versions of GRUB.

For example, GRUB is to be installed to the MBR and some other bootloader (be it GRUB or LILO) is already installed to the boot sector of (hd0,2).

Selecting this entry at boot will load the other distribution's GRUB2 menu assuming that the distribution is installed on /dev/sda3.

Bootloader installation

Manual recovery of GRUB libs

The *stage* files are expected to be in /boot/grub, which may not be the case if the bootloader was not installed during system installation or if the partition/filesystem was damaged, accidentally deleted, etc.

Manually copy the GRUB libs like so:

# cp -a /usr/lib/grub/i386-pc/* /boot/grub

Note: Do not forget to mount the system's boot partition if your setup uses a separate one! The above assumes that either the boot partition resides on the root filesystem or is mounted to /boot on the root file system!

General notes about bootloader installation

GRUB may be installed from a separate medium (e.g. a LiveCD), or directly from a running Arch install. The GRUB bootloader is seldom required to be reinstalled and installation is not necessary when:

Be sure that your GRUB configuration is correct (/boot/grub/menu.lst) before proceeding. Refer to #Finding GRUB's root to ensure your devices are defined correctly.

GRUB must be installed on the MBR (first sector of the hard disk), or the first partition of the first storage device to be recognized by most BIOSes. To allow individual distributions the ability to manage their own GRUB menus, multiple instances of GRUB can be used, see #chainloader and configfile.

Installing the GRUB bootloader may need to be done from within a chrooted environment (i.e. from installed environment via a separate medium) for cases like RAID configurations or if you forgot/broke your GRUB installation. You will need to Change Root from a LiveCD or another Linux installation to do so.

First, enter the GRUB shell:

# grub

Use the root command with the output from the find command (see #Finding GRUB's root) to instruct GRUB which partition contains stage1 (and therefore, /boot):

grub> root (hd1,0)

Tip: The GRUB shell also supports tab-completion. If you type 'root (hd' then press Template:Keypress twice you will see the available storage devices, this can also be done for partitions. Tab-completion also works from the GRUB boot menu. If there is an error in your configuration file you can edit in the boot menu and use tab-completion to help find devices and partitions. See #Edit GRUB entries in the boot menu.

Alternate method (grub-install)

Note: This procedure is known to be less reliable, the recommended method is to use the GRUB shell.

Use the grub-install command followed by the location to install the bootloader. For example to install the GRUB bootloader to the MBR of the first drive:

# grub-install /dev/sda

GRUB will indicate whether it successfully installs. If it does not, you will have to use the GRUB shell method.

Tips and tricks

Additional configuration notes.

Graphical boot

For those desiring eye candy, see grub-gfx. GRUB2 also offers enhanced graphical capabilities, such as background images and bitmap fonts.

Framebuffer resolution

One can use the resolution given in the menu.lst, but you might want to use your LCD wide-screen at its full native resolution. Here is what you can do to achieve this:

On Wikipedia, there is a list of extended framebuffer resolutions (i.e. beyond the ones in the VBE standard). But, for example, the one I want to use for 1440x900 (vga=867) does not work. This is because the graphic card manufacturers are free to choose any number they wish, as this is not part of the VBE 3 standard. This is why these codes change from one card to the other (possibly even for the same manufacturer).

So instead of using that table, you can use one of the tools mentioned below to get the correct code:

GRUB recognized value

This is an easy way to find the resolution code using only GRUB itself.

On the kernel line, specify that the kernel should ask you which mode to use.

kernel /vmlinuz-linux root=/dev/sda1 ro vga=ask

Now reboot. GRUB will now present a list of suitable codes to use and the option to scan for even more.

You can pick the code you would like to use (do not forget it, it is needed for the next step) and boot using it.

Now replace ask in the kernel line with the correct one you have picked.

Note: vbetest gives you VESA mode to which we need to add 512 to get the correct value to use in kernel option line. While hwinfo gives you directly the correct value needed by the kernel.

vbetest

Install the lrmiAUR package from the AUR that contains the vbetest tool (x86_64 users will need to use #hwinfo above).

Run vbetest as root

Then note the number in [ ] corresponding to your desired resolution.

Press 'q' to quit vbetest interactive prompt.

As an option, in a console as root, you can test the mode you just picked up by running vbetest -m <yourcode> and see a pattern like this one

Add 512 to the discovered value picked up above and use the total value to define the vga= parameter in the kernel options of menu.lst.

Reboot to enjoy the result

For example vbetest on one computer:

[356] 1440x900 (256 color palette)
[357] 1440x900 (8:8:8)

So here the number you want is 357. Then, 357 + 512 = 869, so you will use vga=869. Add your value to the end of the kernel line in menu.lst as shown below:

kernel /vmlinuz-linux root=/dev/sda1 ro vga=869

Note:

(8:8:8) is for 24-bit color (24bit is 32bit)

(5:6:5) is for 16-bit color

(5:5:5) is for 15-bit color

Naming partitions

By Label

If you alter (or plan to alter) partition sizes from time to time, you might want to consider defining your drive/partitions by a label. You can label ext2, ext3, ext4 partitions by:

e2label </dev/drive|partition> label

The label name can be up to 16 characters long but cannot have spaces for GRUB to understand it. Then define it in your menu.lst:

kernel /boot/vmlinuz-linux root=/dev/disk/by-label/Arch_Linux ro

By UUID

The UUID (Universally Unique IDentifier) of a partition may be discovered with blkid or ls -l /dev/disk/by-uuid. It is defined in menu.lst with either:

kernel /boot/vmlinuz-linux root=/dev/disk/by-uuid/<uuid number>

or

kernel /boot/vmlinuz-linux root=UUID=<uuid number>

Boot as root (single-user mode)

At the boot loader, select an entry and edit it ('e' key). Append the following parameters to the kernel options:

[...] single init=/bin/bash

This will start in single-user mode (init 1), i.e. you will end up to a root prompt without being asked for password.
This may be useful for recovery features, like resetting the root password.
However, this is a huge security flaw if you have not set any password protection for grub.

Password protection

You can enable password protection in the GRUB configuration file for operating systems you wish to have protected. Bootloader password protection may be desired if your BIOS lacks such functionality and you need the extra security.

Warning: If you disable booting from other boot devices (like a CD drive) in the BIOS's settings and then password protect all your operating system entries, it could be difficult to re-enable booting back into the operating systems if the password is forgotten.

It is always possible to reset your BIOS settings by setting the appropriate jumper on the motherboard (see your motherboard's manual, as it is specific to every model). So in case other have access to the hardware, there is basically no way to prevent boot breakthroughs.

Restart with named boot choice

If you realize that you often need to switch to some other non-default OS (e.g. Windows) having to reboot and wait for the GRUB menu to appear is tedious. GRUB offers a way to record your OS choice when restarting instead of waiting for the menu, by designating a temporary new default which will be reset as soon as it has been used.

Arch is the default (0). We want to restart in to Windows. Change default 0 to default saved -- this will record the current default in a default file in the GRUB directory whenever the savedefault command is used. Now add the line savedefault 0 to the bottom of the Windows entry. Whenever Windows is booted, it will reset the default to Arch, thus making changing the default to Windows temporary.

Now all that is needed is a way to easily change the default manually. This can be accomplished using the command grub-set-default. So, to reboot into Windows, enter the following command:

$ sudo grub-set-default 1 && sudo shutdown -r now

For ease of use, you might to wish to implement the "Allow users to shutdown fix" (including /sbin/grub-set-default amongst the commands the user is allowed to issue without supplying a password).

LILO and GRUB interaction

as some tasks (e.g. kernel compilation using make all) will make a LILO call, and LILO will then be installed over GRUB. LILO may have been included in your base system, depending on your installer media version and whether you selected/deselected it during the package selection stage.

Note: pacman -R lilo will not remove LILO from the MBR if it has been installed there; it will merely remove the lilo package. The LILO bootloader installed to the MBR will be overwritten when GRUB (or another bootloader) is installed over it.

GRUB boot disk

First, format a floppy disk:

fdformat /dev/fd0
mke2fs /dev/fd0

Now mount the disk:

mount -t ext2 /dev/fd0 /mnt/fl

Install GRUB to the disk:

grub-install --root-directory=/mnt/fl '(fd0)'

Copy your menu.lst file to the disk:

cp /boot/grub/menu.lst /mnt/fl/boot/grub/menu.lst

Now unmount your floppy:

umount /mnt/fl

Now you should be able to restart your computer with the disk in the drive and it should boot to GRUB. Make sure that your floppy disk is set to have higher priority than your hard drive when booting in your BIOS first, of course.

Hide GRUB menu

The hiddenmenu option can be used in order to hide the menu by default. That way no menu is displayed and the default option is going to be automatically selected after the timeout passes.
Still, you are able to press Template:Keypress and the menu shows up. To use it, just add to your /boot/grub/menu.lst:

Advanced Debugging

Troubleshooting

GRUB Error 17

Note: the solution below works also for GRUB Error 15

The first check to do is to unplug any external drive. Seems obvious, but sometimes we get tired ;)

If your partition table gets messed up, an unpleasant "GRUB error 17" message might be the only thing that greets you on your next reboot. There are a number of reasons why the partition table could get messed up. Commonly, users who manipulate their partitions with GParted -- particularly logical drives -- can cause the order of the partitions to change. For example, you delete /dev/sda6 and resize /dev/sda7, then finally re-create what used to be /dev/sda6 only now it appears at the bottom of the list, /dev/sda9 for example. Although the physical order of the partitions/logical drives has not changed, the order in which they are recognized has changed.

Fixing the partition table is easy. Boot from your Arch CD/DVD/USB, login as root and fix the partition table:

# fdisk /dev/sda

Once in disk, enter e[x]tra/expert mode, [f]ix the partition order, then [w]rite the table and exit.

You can verify that the partition table was indeed fixed by issuing an fdisk -l. Now you just need to fix GRUB. See the #Bootloader installation section above.

Basically you need to tell GRUB the correct location of your /boot then re-write GRUB to the MBR on the disk.

Accidental install to a Windows partition

If you accidentally install GRUB to a Windows partition, GRUB will write some information to the boot sector of the partition, erasing the reference to the Windows bootloader. (This is true for NTLDR the bootloader for Windows XP and earlier, unsure about later versions).

To fix this you will need to use the Windows Recovery Console for your Windows release. Because many computer manufacturers do not include this with their product (many choose to use a recovery partition) Microsoft has made them available for download. If you use XP, look at this page to be able to turn the floppy disks to a Recovery CD. Boot the Recovery CD (or enable Windows Recovery mode) and run fixboot to repair the partition boot sector. After this, you will have to install GRUB again---this time, to the MBR, not to the Windows partition---to boot Linux.

device.map error

If an error is raised mentioning /boot/grub/device.map during installation or boot, run:

# grub-install --recheck /dev/sda

to force GRUB to recheck the device map, even if it already exists. This may be necessary after resizing partitions or adding/removing drives.

KDE reboot pull-down menu fails

If you have opened a sub-menu with the list of all operating systems configured in GRUB, selected one, and upon restart, you still booted your default OS, then you might want to check if you have the line:

default saved

in /boot/grub/menu.lst.

GRUB fails to find or install to any virtio /dev/vd* or other non-BIOS devices

I had trouble installing GRUB while installing Arch Linux in an virtual KVM machine using a virtio device for hard drive. To install GRUB, I figured out the following:
Enter a virtual console by typing Template:Keypress or any other F-key for a free virtual console.
This assumes that your root file system is mounted in the folder /mnt and the boot file system is either mounted or stored in the folder /mnt/boot.

1. Assure that all needed GRUB files is present in your boot directory (assuming it is mounted in /mnt/boot folder), by issuing the command:

# ls /mnt/boot/grub

2. If the /mnt/boot/grub folder already contains all the needed files, jump to step 3. Otherwise, do the following commands (replacing /mnt, your_kernel and your_initrd with the real paths and file names). You should also have the menu.lst file written to this folder:

4. Enter the following commands. Replace /dev/vda, and (hd0,0) with the correct device and partition corresponding to your setup.

device (hd0) /dev/vda
root (hd0,0)
setup (hd0)
quit

5. If GRUB reports no error messages, then you probably are done. You also need to add appropriate modules to the ramdisk. For more information, please refer to the KVM guide on Preparing an (Arch) Linux guest